best way to add new items to a select?

hey all, I have a select tag like this:

<%= select 'movie','director_id',Director.find(:all).collect {|x| [,]} %>

I would like to add a new director in case it's missing from the select. I would like to do this the Ajax way maybe with a in_place_select? or by putting an rjs link that replace the select with a textfield and a button but I'm not sure maybe there's a better way. I also like how they do it in google mail, where the last option of the select is a "create a new label", then if I click on that option in the select it popups a dialog and then updates the select.

Any ideas or links to demo?

thanx in advance